Recently, opportunistic usage of licensed frequency bands has been proposed as a solution to spectral crowding problems by using cognitive radio (CR) systems. A cognitive radio system would be able to operate in licensed bands by utilizing vacant parts of these bands. A key point for the success of cognitive radio is the ability to shape its signal spectrum to achieve minimum interference to licensed users.
Orthogonal Frequency Division Multiplexing (OFDM) has been proposed as a candidate signaling technology for cognitive radio applications. By dividing the spectrum into subbands that are modulated with orthogonal subcarriers, OFDM spectrum can be shaped more easily in comparison to other signaling techniques known in the art. However, modulated OFDM subcarriers suffer from high sidelobes, which result in adjacent channel interference (ACI). Thus, disabling a set of OFDM subcarriers to create a spectrum null may not be sufficient to avoid interference to licensed users. On the other hand, using filters to reduce the adjacent channel interference can increase the system complexity and introduce long delays. Using guard bands on both sides of used OFDM spectrum coupled with windowing of the time-domain symbols has been investigated. Other proposed methods include the use of interference cancellation carriers, subcarrier weighting, or multiple-choice sequences. While cancellation carrier techniques can significantly suppress OFDM sidelobes, they result in an increase in the system peak-to-average-power ratio (PAPR), and the performance is sensitive to the cyclic prefix (CR) size. Moreover, due to the higher power used for the cancellation carrier method, using this technique affects the spectral flatness of the transmitted signal and can increase the inter-carrier interference (ICI) in case of a Doppler spread of a frequency offset error at the receiver. On the other hand, subcarrier weighting methods cause an increase in the system bit error rate (BER), and the interference reduction is not as significant as it is with the carrier cancellation method.
In the prior art, windowing of OFDM symbols was investigated as a method to suppress OFDM sidelobes. With this process, the time domain symbols are passed through a filter (usually raised cosine (RC) filters are used), and consecutive symbols are allowed to overlap. The process smoothes the transition between OFDM symbols and thus improves the spectral characteristics of the OFDM signal. To keep the orthogonality between OFDM subcarriers, the symbols are cyclicly extended to cover the overlapping region. The advantage of this approach is its low computational complexity. The disadvantage is the reduced spectral efficiency due to the symbol extension.
Accordingly, what is needed in the art is an improved system and method to suppress OFDM sidelobes and shape the signal spectrum for use in cognitive radio applications.